1. Field of the Invention
The invention generally relates to a driving system of a motor vehicle, and in particular to a vehicular driving system that is easily applicable to a plurality of types of driving power sources having different output characteristics.
2. Description of Related Art
A driving system of a motor vehicle is known which is arranged to transmit rotary power generated by a driving power source to drive wheels while changing the speed of rotation by a belt-and-pulley type continuously variable transmission (CVT). Japanese Laid-open Patent Publication No. 11-182666 discloses one example of such a driving system in which a forward-drive/reverse-drive switching device of a planetary gear type is provided between the driving power source and the belt-and-pulley type continuously variable transmission. In operation, the forward-drive/reverse-drive switching device is placed in a selected one of a cut-off mode in which power transmission is cut off, a forward drive mode in which the vehicle runs forward, and a reverse drive mode in which the vehicle runs backward. In the forward drive mode that occurs most frequently, the forward-drive/reverse-drive switching device is rotated as a unit with a clutch thereof being engaged, for a reduction of power loss caused by rotation of meshing gears of the switching device.
In the meantime, a gasoline engine as one type of driving power source used in the vehicular driving system and a diesel engine as another type of driving power source exhibit different output characteristics, for example, provide the maximum power in different rotational speed regions. Generally, the diesel engine provides the maximum power at a lower rotational speed (for example, about 3000–4000 rpm) than the gasoline engine, and therefore the torque of the diesel engine is greater than that of the gasoline engine if the maximum power is at the same level. The increase in the torque makes it necessary to change the design of the belt-and-pulley type continuously variable transmission depending upon the output characteristics of the driving power source used in the system. For example, the size of the continuously variable transmission is increased so that a transmission belt of the CVT is clamped or gripped with an increased belt, clamping force. Also, the diesel engine experiences relatively large torque variations, which makes it necessary to set the belt clamping force to be relatively large in view of the torque variations. For these reasons, power loss in the driving system using the diesel engine as the driving power source is generally larger than that in the system using the gasoline engine.